During operation of a power amplifier circuit comprising a Radio Frequency (RF) power amplifier, it is desirable to achieve relatively high amplifier efficiency across desired power levels (power modes). However, when considering a constant envelope modulated RF signal supplied to such a power amplifier circuit, the signal provided to the power amplifier can vary due to varying operating conditions of the circuit (i.e. temperature and supply voltage). Furthermore, with varying power level requirement from the power amplifier, suitable operating efficiency cannot be readily achieved. For any required operating power level, drain supply and the amplitude of the constant envelope modulated radio frequency signal at an amplifier input must be carefully selected and ideally maintained during circuit operation.